Foreign type tests not covering all operations

[bjorn3]

For example foreign type pointer comparison is not tested. Also &(*val).0 where field 0 of *val is a foreign type is not tested. As a last thing pointer to *mut ForeignType casting it not tested.

These were all cases which were mishandled in rustc_codegen_cranelift. I didn't discover them until I tried to compile rustc itself using cg_clif.

[bjorn3]

I am currently using

extern "C" {
    type ExternType;
}

struct ExternTypeWrapper {
    _a: ExternType,
}

let nullptr = 0 as *const ();
let extern_nullptr = nullptr as *const ExternTypeWrapper;
extern_nullptr as *const ();
let slice_ptr = &[] as *const [u8];
slice_ptr as *const u8;

as test for cg_clif.